1. Technical Field
The present disclosure is related to fuel systems and the routing of return fuel from the fuel injectors and fuel rail(s).
2. Background Art
A variety of fuel systems are commonly used on diesel engines, including: common rail systems, mechanical and hydraulic unit injectors, and other systems that employ a low pressure feed pump supplying pressurized flow to the high pressure fuel system. The fuel injection event on these systems, commanded by the engine's electronic control unit, actuates the injector to allow fuel to spray through injector orifices into the combustion chamber. The injectors are designed such that a portion of the fuel delivered to the injectors is used to actuate the injector; the portion of fuel that used to actuate the injector is returned to a lower pressure portion of the fuel system.
In many prior art systems, fuel from the injector is returned to the fuel tank through return lines coupled to each injector. This reduces overall system efficiency because the low pressure feed pump(s) must pressurize the fuel again and deliver it to the injectors for subsequent combustion cycles. In addition, returning all the fuel to the tank will require a higher capacity pump.
Various types of injectors, such as piezoelectric injectors, do not operate well in the absence of a residual or priming pressure within the injector to bias internal components toward a desired starting position. These systems are more difficult to start when the return lines are coupled to the fuel tank as the residual pressure is essentially atmospheric pressure, which allows internal components to “collapse.” The problem is exacerbated when air is ingested in the fuel system. Ingested air is very difficult to purge without injectors operating properly.
In other prior art, a return line from a fuel injector is coupled to a low pressure line between a high pressure pump and a low pressure fuel supply pump (or lift pump). In the prior art, two fine filters are provided, one upstream of where the return line tees into the low pressure line and one in the fuel return line. Each filter is replaced on a particular interval, which involves additional cost to maintain and replace. Furthermore, each component in the system has connectors which can present an opportunity for leakage: during operation if not properly secured, during operation due to loosening or damage, and during maintenance.
The fuel returned from the fuel injectors and common rail is heated by pressurization as well as its proximity to hot engine components. The fuel supplied from the low pressure fuel pump comes from the fuel tank and is closer to ambient temperature. The two streams of fuel are combined but little mixing ensues before the fuel enters the high pressure pump. This may result in slugs of fuel of different temperatures being provided to the high pressure pump. When the pump is provided a slug of the hotter fuel, it can exceed the maximum design temperature of the high pressure fuel pump. Furthermore, pump durability may be adversely impacted by rapidly changing fuel temperatures associated with insufficient mixing of return fuel and tank fuel. According to the prior art configuration with return fuel teeing into the low pressure line in between high and low pressure pumps, such inadequate mixing results.